Continuous hot dip galvanizing of metal strip



Sept. 13, 1960 w. L. DIEHL ETAL 2,952,568

ONTINUOUS HOT DIP GALVANIZING OF METAL STRIP Original Filed Aug. 19,1955 INVENTORS William L. Diehl 8 Henry 8. Bell 9 7&- 0% k CONTINUOUSHOT DIP GALVANIZING OF METAL STRIP William L. Diehl, St. Clair-sville,Ohio, and Henry S. Bell, Wheeling, W Va., assignors to Wheeling SteelCorporation, Wheeling, W. Va., a corporation of Delaware Originalapplication Aug. 19, 1955, Ser. No. 529,562,

new Patent No. 2,875,096, dated Feb. 24, 1959. Di- ;i2ded4and thisapplication Mar. 26, 1958, Ser. No.

1 Claim. (Cl. 117-102) This invention relates to the continuous hot dipgalvanizing of metal strip and more particularly to an improved processwhich contributes importantly to the ability to produce galvanized stripof optimum quality at unprecedented speed. This application is adivision of our copending application Serial No. 529,562, filed August19, 1955, now Patent No. 2,875,096.

As continuous line speeds have increased during the past decade or sogreat difiiculty has been experienced in producing by continuous hot dipgalvanizing galvanized strip of uniform optimum quality. The increasedspeed of the strip produces a strong tendency of the strip to carry withit an excessive quantity of spelter as the strip emerges from the bathof molten spelter and there is also a tendency for the coating on thegalvanized strip to be relatively thick near the edges of the strip andrelatively thin near the center of the strip.

It is possible by the use of grooved exit or coating rolls to counteractto some extent the above mentioned tendencies, but for the higher speedswhich are now commercial grooved exit rolls do not adequately serve thepurpose. Moreover, while for some intermediate speeds grooved exit rollsmight be designed which would produce galvanized strip of optimumquality for a particular strip speed in the application of a particularweight of coating to a strip of a particular gauge, such rolls would notbe satisfactory if the conditions should be substantially changed.

We have found that the tendency of the strip to carry an excessiveamount of molten spelter up out of the bath when the strip speedincreases can be counteracted or compensated for by regulating the driveof the exit rolls so that the exit rolls inhibit the movement ofspelterupwardly between the rolls relatively to the effect of the exitrolls upon upward movement of the spelter when the exit rolls arenormally driven. We reduce the speed at which the exit rolls are driven.The speed at which the exit rolls should be driven depends upon thespeed and gauge of the strip and the thickness of coating desired.Generally speaking, the faster the speed of the strip and the thinnerthe coating desired the greater should be the difference between thespeed of the strip and the speed of the exit rolls. The exit rollsshould be driven at a speed somewhat slower than the speed of the strip.

Another factor which causes difiiculty is the capillary attraction ofthe strip for the spelter. The exit rolls will break down the capillaryattraction to a certain extent but their ability to do so is lessened asthe strip speed and coating thickness increase. We have found it highlydesirable to provide means engaging the strip in the molten spelter bathbefore the strip reaches the exit rolls to inhibit the tendency ofspelter to cling to the strip, or, in other words, to break down thecapillary attraction of the strip for the spelter. We desirably bringrolls to bear against the respective faces of the strip in the bathbefore the strip reaches the exit rolls.

The means engaging the strip in the bath before the strip reaches theexit rolls to inhibit the tendency of spelter to cling to the stripshould be so positioned relatively to the exit rolls that substantialcapillary attraction of the spelter to the strip cannot be set upbetween said means and the exit rolls. If such means are spaced too faraway from the exit rolls capillary attraction between the strip and thespelter may be set up all over again be fore the strip reaches the exitrolls even though it has been reduced by such means. We prefer to employopposed rolls in the bath arranged to bear against the respective facesof the strip before the strip reaches the exit rolls and therebyinhibiting the tendency of spelter to cling by capillary attraction tothe strip, such opposed rolls being so positioned relatively to the exitrolls that substantial capillary attraction of the spelter to the stripcannot be set up between such rolls and the exit rolls. The capillarityreducing rolls desirably have faces resistant to wetting by moltenspelter. Such rolls may be made of stainless steel or they maybe'chromium plated or surfaced with some hard, glassy material resistantto wetting by molten spelter. Materials which are resistant to wettingby molten spelter are known to those skilled in the art.

The capillarity reducing rolls are preferably relatively resilientlyurged toward the strip. One of those rolls may be mounted for rotationabout a fixed axis and the other may be resiliently pressed toward theroll which is mounted for rotation about a fixed axis.

The distance between the capillarity reducing rolls and the exit orcoating rolls depends upon the speed of the strip, the gauge of thestrip and the thickness of the galvanized coating being applied to thestrip. It is not possible to state empirically what that distance shouldbe just as it is not possible to state empirically just what the speedof the exit rolls should be since the variables which have beenmentioned have to be taken into consideration. However, a person skilledin the art following our teaching will have no ditficulty ascertainingthe optimum speed the optimum distance between the capillarity reducingrolls and the exit rolls for a particular strip speed coupled with aparticular strip gauge and a particular thickness of coating.

Either grooved or ungrooved exit rolls may be used. For ungrooved rollsthe speed differential between the rolls and the strip for high stripspeeds may be somewhat less than when grooved rolls are employed.

The capillarity reducing rolls may be pressed together by any convenientmeans, the purpose being to press those rolls against the strip withsutficient pressure that as the strip advances it will cause thecapillarity reducing rolls to turn by engagement with the strip. Thusnormally the capillarity reducing rolls, being driven by the strip, turnat a peripheral speed substantially equal to the linear speed of thestrip.

The capillarity reducing rolls are preferably of smaller diameter thanthe exit rolls. We have found that generally speaking it is desirable tohave the capillarity reducing rolls not greater than about three inchesin diameter. They are preferably ungrooved and as: above statedpreferably have faces resistant to wetting by molten spelter. Forexample, the capillarity reducing rolls may be made of, or at leastfaced with, alloy steel.

For very high speeds it may be desirable to provide capillarity reducingrolls in tandem, i.e., to provide more than one set of such rollsengaging the strip successively as the strip advances through thespelter bath toward the exit rolls.

Other details, objects and advantages of the invention will becomeapparent as the following description of a present preferred method ofpracticing the same proceeds.

In the accompanying drawing we have shown apparatus capable of use inpracticing the invention, the figure being a fragmentary view partly inelevation and partly in vertical cross-section of a hot dip galvanizingpot. We have found that when the exit rolls of a galvanizing pot arerotated so that their peripheral speed equals the linear speed of thestrip being galvanized there is a critical speed range beyond which thecoating cannot be effectively controlled either as to weight or as toquality due to the fact that the increased strip speed tends to carry anexcess of molten spelter into the exit rolls, largely by capillarity,which excess spelter cannot be properly distributed over the exit rollsand the strip. At times streams of excess coating metal andimperfections occur on the coated strip and the longitudinal edges ofthe strip may be relatively heavily coated due to the ex- 'cess ofmolten spelter carried up by the exit rolls. We have found that thecritical speed above referred to is substantially in the range 140-l 60feet per minute. The critical speed varies with the gauge of the stripbeing coated, the composition of the spelter bath, the fluidity of thebath, the temperature of the bath, the thickness of thecoating beingapplied to the strip, etc.

Continuous galvanizing lines are now operated at speeds above. 300 feetper-minute and speeds in'the range 500 600 feet per minute are probablein the near' future. For strip speeds in excess of the critical rangeabove referred to we drive the exit rolls at speeds different from thestrip speed and/or'provide for breaking the capillarity of the moltenrnetalbefore the strip reaches the exit rolls to reduce the amount ofmolten spelter carried up bythe strip. I

When the stripspeed is inexcess of the above mentioned critical speed weprefer to operate the exit rolls at aperipheral speed in the range 3060feet per minute. All of the speeds mentioned aretsubject to variationdepending upon the variations in operating conditions as referred toabove.

Referring 'now more particularly to the drawing, there is shown agalvanizing pot designated generally by reference numeral 2 and whichmay be of conventional construction. It is adapted to contain moltenspelter, the level of the molten spelter in the galvanizing'pot 2 beingindicated by the line 3. By spelter We mean any molten bath suitable forgalvanizing and which is primarily zinc. The spelter may besubstantially all zinc or it may have other metals admixed with thezinc, such, for example, as aluminum, lead, antimony, cadmium, tin, andso on, either singly or in various combinations.

The exit rolls, sometimes called coating rolls, are shown at 4,.beingmounted in conventional manner partly immersed in the molten spelter andpartly extending above the surface of the molten spelter. As shown inthe drawing the surface of the molten spelter is substantially at theelevation of the axes .of the exit rolls .4. The exit rolls are providedwith driving means comprising intermeshing, gears 14.driven by a drivinggear 15 on a driving shaft 16.

The strip enters the galvanizing pot in conventional manner and passesthrough the molten spelter and about a guide roll 5 rotatably mountedbelow the surface of the spelter, the strip, which is designated byreference numeral :6, passing vertically upwardly from the guide roll 5between the exit rolls 4 as shown.

We provide opposed eapillarity reducing rolls 7 and 8 adapted to engagethe strip at opposite faces thereof as shown in the drawing. Thecapillarity reducing rolls 7 and 8 have faces resistant to wetting bymolten spelter and are relatively resiliently urged toward the strip. Inthe form shown the roll 7 is mounted for rotation about a fixed axis andthe roll 8 is resiliently pressed toward the roll 7 The roll 8 isrotatably mounted in a pivoted mounting structure 9 which is pivoted at10 to a stationarily mounted bracket 11. Connected with the mountingstructure 9 is an outwardly extending arm 12 carrying a weight 13 whichis adjustably positionable along the arm. The structure 9, the arm 12and the weight 13 are to all intents and purposes unitary and the weightmay be adjusted along the arm as desired to resiliently press the roll 8toward the roll 7 with the desired force. The rolls 7 and 8 are bothfreely rotatable and are turned by the movement of the strip in contacttherewith.

The rolls 7 and 8 are so positioned relatively to the exit rolls 4 thatsubstantial capillary attraction of the spelter to the strip cannot beset up between the rolls 7 and 8 and the rolls 4. The rolls 7 and 8reduce or break the capillary attraction of the spelter forthe strip andthus cooperate with the exit rolls 4 in controlling 'the application ofthe spelter to the strip.

As mentioned above, the distance between the rolls 4 and the rolls 7 and8 cannot be empirically stated and depend upon the speed of the strip,the gauge of the strip and the thickness of the coating applied to thestrip. However, by slowing down the speed of the rolls 4 .relatively tothe strip and maintaining the distance between the rolls -4 and therolls 7 and 8 as above explained galvanized strip of optimum quality canbe produced at unprecedented speed. i

While we have illustrated and described a present preferred method ofpracticing the invention it is to be distinctly understood that theinvention is not limited thereto but may be otherwise practiced withinthe scope of the following claim.

We claim: 1 a, I In the continuous hot dip galvanizing of metal strip inwhich the strip is delivered upwardly at high speed out of thegalvanizing bath of molten spelter between exit rolls partly immersed inthe bath engaging the respective facesof the strip and arranged withtheir axes parallel and generallyhorizontal and which exit rolls arenormally driven with their portions next the strip'moving upwardly, thestep of inhibiting the carrying up out of the bath of an excessiveamount of molten spelter which comprises positively driving the exitrolls in the direction of advance of the stripat a speed somewhat slowerthan the speed of the strip whereby the exit rolls exert a drag on themolten spelter uniformly across the strip and at the edges and therebyrestrain movement of spelter upwardly between the rolls relatively tothe effect of the exit rolls upon upward movement of the spelter whenthe exit rolls are driven at a speed equal to the speed of the strip.

References Cited in the file of this patent f UNITED STATES PATENTS2,364,904 Keller Dec. 12,1944 2,377,632 Keller June 5, 1945 x 2,401,374Sendzimir June 4, 1946 2,740,729 Hodil Apr. 3, 1956 2,797,476 SendizimerJuly 2, 1957 2,875,096 Diehl et al. Feb. 24, 1959 OTHER REFERENCESBablic: Galvanizing (Hot Dip), page 434, 3rd ed., 1950; Spon Ltd.,London. 7

